Cooling passages for a mid-turbine frame

ABSTRACT

A mid-turbine frame includes an inner frame case including a fluid passage. At least one hollow spoke is attached to the inner frame case. At least one hollow spoke includes an inlet passage. A transfer tube includes a first end that forms a first connection with at least one hollow spoke and a second end forms a second connection with the inner frame case.

BACKGROUND

The present disclosure relates generally to a gas turbine engine, and in particular to a mid-turbine frame (MTF) included in a gas turbine engine.

A gas turbine engine typically includes a fan section, a compressor section, a combustor section, and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section where it is mixed with fuel and ignited to generate a high-speed exhaust gas flow. The high-speed exhaust gas flow expands through the turbine section to drive the compressor and the fan section.

A mid-turbine frame (MTF) is positioned between a high pressure turbine stage and a low pressure turbine stage of a gas turbine engine. The MTF supports one or more bearings and transfers bearing loads from an inner portion of the gas turbine engine to an outer engine frame. The MTF also serves to route air from the high pressure turbine stage to the low pressure turbine stage.

SUMMARY

In one exemplary embodiment, a mid-turbine frame includes an inner frame case including a fluid passage. At least one hollow spoke is attached to the inner frame case. At least one hollow spoke includes an inlet passage. A transfer tube includes a first end that forms a first connection with at least one hollow spoke and a second end forms a second connection with the inner frame case.

In a further embodiment of the above, the first connection includes a spherical joint.

In a further embodiment of any of the above, the spherical joint includes a concave surface on one of the transfer tube and at least one hollow spoke. A cylindrical surface is on the other of the transfer tube and at least one hollow spoke.

In a further embodiment of any of the above, the first connection includes a slip joint with one of the transfer tube and at least one hollow spoke being received in the other of the transfer tube and at least one hollow spoke.

In a further embodiment of any of the above, the first connection is configured to allow relative axial and pivoting movement between at least one hollow spoke and the transfer tube.

In a further embodiment of any of the above, the second connection is configured to rigidly secure the transfer tube to the inner frame case.

In a further embodiment of any of the above, the transfer tube includes a flange and a neck portion that extend from the flange.

In a further embodiment of any of the above, the transfer tube includes a passage with a circular cross section at a first end and a race track cross section at a second end.

In another exemplary embodiment, a gas turbine engine includes a mid-turbine frame located axially between a first turbine and a second turbine. The mid-turbine frame includes an inner frame case including a fluid passage. At least one hollow spoke is attached to the inner frame case. At least one hollow spoke includes an inlet passage. A transfer tube includes a first end that forms a first connection with the at least one hollow spoke and a second end that forms a second connection with the inner frame case.

In a further embodiment of any of the above, the first connection includes a spherical joint. The spherical joint includes a concave surface on one of the transfer tube and the at least one hollow spoke and a cylindrical surface on the other of the transfer tube and the least one hollow spoke.

In a further embodiment of any of the above, the first connection includes a slip joint with one of the transfer tube and the at least one hollow spoke is received in the other of the transfer tube and at least one hollow spoke.

In a further embodiment of any of the above, the first connection is configured to allow relative movement between at least one hollow spoke and the transfer tube.

In a further embodiment of any of the above, the second connection is configured to rigidly secure the transfer tube to the inner frame case.

In a further embodiment of any of the above, the transfer tube includes a flange and a neck portion that extends from the flange.

In a further embodiment of any of the above, the transfer tube includes a passage with a circular cross section at a first end. One of a race track cross section, circular cross section, and elliptical cross section at a second end.

In a further embodiment of any of the above, the inlet passage in at least one hollow spoke is in fluid communication with a low-rotor cavity.

In another exemplary embodiment, a method of assembling a mid-turbine frame includes forming a first connection between at least one hollow spoke and a transfer tube. A second connection is formed between the transfer tube and an inner frame case. One of the first connection and the second connection is rigid and the other of the first connection and the second connection is non-rigid.

In a further embodiment of any of the above, the first connection includes a spherical joint. The spherical joint includes a concave surface on one of the transfer tube and at least one hollow spoke. A convex surface on the other of the transfer tube and at least one hollow spoke.

In a further embodiment of any of the above, the first connection includes a slip joint with one of the transfer tube. At least one hollow spoke is received in the other of the transfer tube and at least one hollow spoke.

In a further embodiment of any of the above, the second connection is configured to rigidly secure the transfer tube to the inner frame case.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an example gas turbine engine.

FIG. 2 is a schematic perspective view of an example mid-turbine frame in the gas turbine engine.

FIG. 3 is a sectional view taken along line 3-3 of FIG. 2.

FIG. 3A is a sectional view of an example joint between an example tie rod and an example transfer tube.

FIG. 3B is a sectional view of another example joint between the example tie rod and another example transfer tube.

FIG. 3C is a sectional view of a further example joint between the example tie rod and a further example transfer tube.

FIG. 4 is a perspective view of sectional view of FIG. 3.

FIG. 5 is a perspective view of the example transfer tube.

FIG. 6 is another perspective view of the example transfer tube.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a gas turbine engine 20. The gas turbine engine 20 is disclosed herein as a two-spool turbofan that generally incorporates a fan section 22, a compressor section 24, a combustor section 26 and a turbine section 28. Alternative engines might include an augmentor section (not shown) among other systems or features. The fan section 22 drives air along a bypass flow path B in a bypass duct defined within a nacelle 15, while the compressor section 24 drives air along a core flow path C for compression and communication into the combustor section 26 then expansion through the turbine section 28. Although depicted as a two-spool turbofan gas turbine engine in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use with two-spool turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures.

The exemplary engine 20 generally includes a low speed spool 30 and a high speed spool 32 mounted for rotation about an engine central longitudinal axis A relative to an engine static structure 36 via several bearing systems 38. It should be understood that various bearing systems 38 at various locations may alternatively or additionally be provided, and the location of bearing systems 38 may be varied as appropriate to the application.

The low speed spool 30 generally includes an inner shaft 40 that interconnects a fan 42, a first (or low) pressure compressor 44 and a first (or low) pressure turbine 46. The inner shaft 40 is connected to the fan 42 through a speed change mechanism, which in exemplary gas turbine engine 20 is illustrated as a geared architecture 48 to drive the fan 42 at a lower speed than the low speed spool 30. The high speed spool 32 includes an outer shaft 50 that interconnects a second (or high) pressure compressor 52 and a second (or high) pressure turbine 54. A combustor 56 is arranged in exemplary gas turbine 20 between the high pressure compressor 52 and the high pressure turbine 54. A mid-turbine frame 57 of the engine static structure 36 is arranged generally between the high pressure turbine 54 and the low pressure turbine 46. The mid-turbine frame 57 further supports bearing systems 38 in the turbine section 28. The inner shaft 40 and the outer shaft 50 are concentric and rotate via bearing systems 38 about the engine central longitudinal axis A which is collinear with their longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 then the high pressure compressor 52, mixed and burned with fuel in the combustor 56, then expanded over the high pressure turbine 54 and low pressure turbine 46. The mid-turbine frame 57 includes airfoils 59 which are in the core airflow path C. The turbines 46, 54 rotationally drive the respective low speed spool 30 and high speed spool 32 in response to the expansion. It will be appreciated that each of the positions of the fan section 22, compressor section 24, combustor section 26, turbine section 28, and fan drive gear system 48 may be varied. For example, gear system 48 may be located aft of combustor section 26 or even aft of turbine section 28, and fan section 22 may be positioned forward or aft of the location of gear system 48.

The engine 20 in one example is a high-bypass geared aircraft engine. In a further example, the engine 20 bypass ratio is greater than about six (6), with an example embodiment being greater than about ten (10), the geared architecture 48 is an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3 and the low pressure turbine 46 has a pressure ratio that is greater than about five. In one disclosed embodiment, the engine 20 bypass ratio is greater than about ten (10:1), the fan diameter is significantly larger than that of the low pressure compressor 44, and the low pressure turbine 46 has a pressure ratio that is greater than about five 5:1. Low pressure turbine 46 pressure ratio is pressure measured prior to inlet of low pressure turbine 46 as related to the pressure at the outlet of the low pressure turbine 46 prior to an exhaust nozzle. The geared architecture 48 may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.3:1. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present invention is applicable to other gas turbine engines including direct drive turbofans.

A significant amount of thrust is provided by the bypass flow B due to the high bypass ratio. The fan section 22 of the engine 20 is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet. The flight condition of 0.8 Mach and 35,000 ft (10,668 meters), with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (‘TSFC’)”—is the industry standard parameter of lbm of fuel being burned divided by lbf of thrust the engine produces at that minimum point. “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.45. “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tram ° R)/(518.7° R)]^(0.5). The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second (350.5 meters/second).

The example gas turbine engine includes fan 42 that comprises in one non-limiting embodiment less than about twenty-six (26) fan blades. In another non-limiting embodiment, fan section 22 includes less than about twenty (20) fan blades. Moreover, in one disclosed embodiment low pressure turbine 46 includes no more than about six (6) turbine rotors schematically indicated at 34. In another non-limiting example embodiment low pressure turbine 46 includes about three (3) turbine rotors. A ratio between number of fan blades 42 and the number of low pressure turbine rotors is between about 3.3 and about 8.6. The example low pressure turbine 46 provides the driving power to rotate fan section 22 and therefore the relationship between the number of turbine rotors 34 in low pressure turbine 46 and number of blades 42 in fan section 22 disclose an example gas turbine engine 20 with increased power transfer efficiency.

FIG. 2 is a schematic perspective view of one embodiment of the mid-turbine frame 57. The schematic view shown in FIG. 2 is high level conceptual view and is intended to illustrate relative positioning of various components, but not actual shape of various components. The mid-turbine frame 57 includes an outer frame case 62, an inner frame case 64, and a plurality of hollow spokes 65. The outer frame case 62 includes an outer diameter surface 66. The inner frame case 64 includes an outer diameter surface 70 and an inner diameter surface 72. In the embodiment shown in FIG. 2, six hollow spokes 65 are distributed around the circumference of the inner frame case 64 to provide structural support between the inner frame case 64 and the outer frame case 62. In alternative embodiments, the mid-turbine frame 57 can have more or less than six hollow spokes.

The inner frame case 64 supports the rotor assembly via the bearing assemblies 38 (shown in FIG. 1), and distributes the force from the inner frame case 64 to the outer frame case 62 via the plurality of hollow spokes 65. Attachment of the hollow spokes 65 to the outer frame case 62 is provided at a plurality of bosses 75 located circumferentially around the outer diameter surface 66 of the outer frame case 62.

In one embodiment, attachment of the hollow spokes 65 at the plurality of bosses 75 may be secured by a retaining nut 106 (shown in FIG. 3) that allows the hollow spokes 65 to be tensioned. The hollow spokes 65 can be tensioned via a threaded connection so as to remain in tension during substantially all operating conditions of gas turbine engine 20. Apertures 76 formed in each of the plurality of bosses 75 allow cooling airflow to be distributed into a hollow portion of each of the hollow spokes 65. In this way, the cooling airflow is directed from the outer diameter through the hollow portions of the cooled hollow spokes 65 towards the inner frame case 64. The cooling airflow can function to cool the hollow spokes 65 and also to cool components radially inward of the inner frame case 64, such as the bearings 38. The cooling airflow is then directed to the low-rotor cavity 126 to cool the turbine rotors.

FIG. 3 is a cross-sectional view of the mid-turbine frame 57 taken along line 3-3 of FIG. 2. A hollow spoke 65A is one example of the hollow spokes 65 shown in FIG. 2. The hollow spoke 65A extends from the outer frame case 62 through the airfoil 59 to the inner frame case 64. The airfoil 59 extends from an outer platform 78 to an inner platform 80. In the illustrated embodiment, the airfoil 59, the outer platform 78, and the inner platform 80 are integrally formed, and are all positioned radially inward of the outer frame case 62 and radially outward of the inner frame case 64. The airfoil 59, the outer platform 78, and the inner platform 80 define a portion of the core flow path C at the mid-turbine frame 57. The airfoil 59 extends axially from a leading edge 82 to a trailing edge 84. The airfoil 59 is oblong so as to be longer in the axial direction than in the circumferential direction. The airfoil 59 has a hollow interior 86, which is also relatively narrow in a circumferential direction.

In the illustrated embodiment, the hollow spoke 65A includes a tie rod 90A and a retaining nut 92. The tie rod 90A is an elongated hollow tube that includes a threaded surface 94 at a radially outer end and a flange 96 at a radially inner end. The threaded surface 94 is on an outer surface 98 of the tie rod 90A. An inner passage surface 100 of the tie rod 90A defines an inlet passage 118 through the tie rod 90A. The tie rod 90A tapers along its length from the flange 96 at its radially inner end to the threaded surface 94 at its radially outer end.

The retaining nut 92 includes a threaded surface 102 at a radially inner end of the retaining nut 92 and a flange 104 at a radially outer end of the retaining nut 92. The threaded surface 102 is on an inner surface 106 of the retaining nut 92. The flange 104 extends outward from an outer surface 108 of the retaining nut 92.

In the illustrated embodiment, the flange 96 of the tie rod 90A abuts against the inner frame case 64 so that the inner passage surface 100 aligns with a hole 110A in the inner frame case 64. The flange 96 is attached to the inner frame case 64 via bolts 112. The retaining nut 92 extends through a hole 114 in the outer frame case 62 such that the flange 104 abuts against the outer diameter surface 66 of the outer frame case 62. The flange 104 is attached to the outer frame case 62 via a bolt 116. The bolt 116 extends through the flange 104 into the outer frame case 62. The tie rod 90A is threaded into the retaining nut 92 to attach the tie rod 90A to the retaining nut 92. In the illustrated embodiment, a portion but not all of the threaded surface 94 overlaps with a portion but not all of the threaded surface 102.

During assembly, the tie rod 90A is inserted through the hollow interior 86 of the airfoil 59 in a direction from radially inward to radially outward. The inner frame case 64 is then positioned radially inward of the tie rod 90A and attached to the tie rod 90A by the bolts 112. The retaining nut 92 is then inserted through the hole 114 and threadedly engaged with the tie rod 90A. The retaining nut 92 can be tightened, as desired, in a manner described below. Once the retaining nut 92 is suitably tightened on the tie rod 90A, the bolt 116 is inserted to fix the retaining nut 92 to the outer frame case 62 to prevent the retaining nut 92 from rotating and loosening.

Because the threaded surface 94 overlaps with the threaded surface 102 only partially, the threaded connection between the retaining nut 92 and the tie rod 90A is variable. The retaining nut 92 does not bottom out at any particular point when threaded on the tie rod 90A. This allows the retaining nut 92 to be threaded on the tie rod 90A to an extent determined during assembly, not predetermined prior to assembly. This allows the hollow spoke 65A, and the mid-turbine frame 57 in general, to be relatively insensitive to manufacturing tolerances.

The inlet passage 118 branches off between a first branch 120 extending into a bearing support cavity 122 and a second branch 124 extending into a low-rotor cavity 126. The bearing support cavity 122 is at least partially defined by the inner frame case 64 and a bearing support member 123. The first branch 120 extends in a radially inward direction through the inner frame case 64.

A plug 128 is aligned with the first branch 120 and is located in an opening 130 in the hollow spoke 65A adjacent the outer diameter surface 70 of the inner frame case 64. The plug 128 includes an opening 129 having a conical radially outer portion that tapers to a cylindrical channel on a radially inner side. The cylindrical channel of the plug 128 includes a diameter D1 that is smaller than a diameter D2 defined by the inner passage surface 100. In the illustrated example, the plug 128 includes a diameter D1, however, the diameter D1 could be any dimension that is smaller than the dimension D2 in order to control the amount of cooling airflow that travels into the bearing support cavity 122. Although the plug 128 is shown contacting the hollow spoke 65 a and the inner frame case 64, the plug 128 could be located anywhere within the first branch 120.

In another example embodiment, the plug 128 could be solid and prevent the cooling airflow from entering the bearing support cavity 122 so the entire cooling airflow must travel through the second branch 124. In yet another example embodiment, the opening 130 in the tie rod 90A could be reduced to the diameter D1 so that the plug 128 could be eliminated.

The second branch 124 extends in an axially downstream direction perpendicular to the first branch 120. Although the second branch 124 is shown being perpendicular to the first branch 120, the second branch 124 could be within 30 degrees of being perpendicular to the first branch 120. The second branch 124 is in fluid communication with the low rotor cavity through to a fitting 132 that extends through the inner frame case 64 and connects to a swirler tube 142.

The fitting 132 includes a transfer tube 134 that directly contacts an opening 138 in the tie rod 90A at a first axial end 135 forming a first connection and contacts the inner frame case 64 at a second axial end 136 forming a second connection. The opening 138 in the tie rod 90A and the first axial end 135 of the transfer tube 134 form a spherical joint. As shown in FIG. 3B, the opening 138 includes a cylindrical hole that contacts a convex surface 170 on a first axial end 135′ of a transfer tube 134′. In another example as shown in FIG. 3A, the first axial end 135 could also be cylindrical and fit into the opening 138 with a small controlled annular gap to form a joint 139.

The spherical joint formed between the tie rod 90A and the first axial end 135′ creates a seal without the use of an O-ring or piston seal. The spherical joint also allows for relative axial and pivoting movement between the tie rod 90A and the transfer tube 134′.

As shown in FIG. 3C, a slip joint could be formed with one of the opening 138 and a first axial end 135″ being received in the other of the opening 138 and the first axial end 135″ with a piston seal 174 optionally located between the opening 138 and first axial end 135″. In one example, the first axial end 135″ of a transfer tube 134″ would include a piston ring slot 172 for capturing the piston seal 174 that would be inserted into the opening 138 to form the slip joint.

FIGS. 5 and 6 show perspective views of the transfer tube 134. As shown in FIG. 5, the first axial end 135 includes a passage 150 extending through a neck portion 154 and a flange 156 of the transfer tube 134. The passage 150 includes an inlet 152 having a circular cross section. As the passage 150 extends towards the second axial end 136 and an outlet 153, the cross section of the passage 150 transitions into a race track shape with rounded end portions connected by parallel sides. The race track shape of the outlet 153 diffuses the cooling airflow allows the flow area to pass through a limited flange height before the cooling airflow reaches the swirler tube 142. In another example, the outlet 153 could have an elliptical or circular cross section.

The flange 156 includes a first side 158 adjacent the neck portion 154 and a second side 159 that is configured to mount against the inner frame case 64. The transfer tube 134 is fastened to the inner frame case 64 with fasteners 140 (FIG. 4) extending through fastener opening 160 in the flange 156.

The passage 150 is aligned with a fluid passage 110B in the inner frame case 64 to allow the cooling airflow to travel from the inlet passage 118 in the tie rod 90A through the passage 150 in the transfer tube 134 into the low-rotor cavity 126. A pair of anti-rotation members 162 engage tabs 164 on the flange 156 of the transfer tube 134 adjacent the neck portion 154 to prevent movement of the transfer tube 134.

The fasteners 140 also secure the swirler tube 142 to an opposite side of the inner frame case 64 from the transfer tube 134. The swirler tube 142 directs the cooling airflow into the low-rotor cavity 126 in the direction of rotation of the low rotor to reduce turning and aerodynamic losses in the cooling airflow.

A restricting ring 144 is located between the swirler tube 142 and the inner diameter surface 72. The restricting ring 144 includes a diameter D3 which is smaller than a diameter D4 of the second branch 124. The restricting ring 144 restricts the amount of cooling airflow through the second branch 124 to aid in dividing the amount of cooling airflow traveling into the bearing support cavity 122 and the low-rotor cavity 126. Although the restricting ring 144 is shown between the swirler tube 142 and the inner frame case 64, the restricting ring 144 could be located anywhere within the second branch 124 to reduce the cooling airflow into the low-rotor cavity 126. Alternatively, or in addition to the restricting ring 144, a portion of the second branch 124 may include a portion with a reduced diameter, such as reducing a diameter of the second branch 124 extending through the transfer tube 134, the boss 136, or the hole 110B to meter the cooling airflow.

The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims. 

What is claimed is:
 1. A mid-turbine frame comprising: inner frame case including a fluid passage; at least one hollow spoke attached to the inner frame case, wherein the at least one hollow spoke includes an inlet passage; and a transfer tube including a first end forming a first connection with the at least one hollow spoke and a second end forming a second connection with the inner frame case.
 2. The mid-turbine frame of claim 1, wherein the first connection includes a spherical joint.
 3. The mid-turbine frame of claim 2, wherein the spherical joint includes a convex surface on one of the transfer tube and the at least one hollow spoke and a cylindrical surface on the other of the transfer tube and the at least one hollow spoke.
 4. The mid-turbine frame of claim 1, wherein the first connection includes a slip joint with one of the transfer tube and the at least one hollow spoke being received in the other of the transfer tube and the at least one hollow spoke.
 5. The mid-turbine frame of claim 1, wherein the first connection is configured to allow relative axial and pivoting movement between the at least one hollow spoke and the transfer tube.
 6. The mid-turbine frame of claim 5, wherein the second connection is configured to rigidly secure the transfer tube to the inner frame case.
 7. The mid-turbine frame of claim 1, wherein the transfer tube includes a flange and a neck portion extending from the flange.
 8. The mid-turbine frame of claim 7, wherein the transfer tube includes a passage with a circular cross section at a first end and a race track cross section at a second end.
 9. A gas turbine engine comprising: a mid-turbine frame located axially between a first turbine and a second turbine, the mid-turbine frame comprising: inner frame case including a fluid passage; at least one hollow spoke attached to the inner frame case, wherein the at least one hollow spoke includes an inlet passage; and a transfer tube including a first end forming a first connection with the at least one hollow spoke and a second end forming a second connection with the inner frame case.
 10. The gas turbine engine of claim 9, wherein the first connection includes a spherical joint and the spherical joint includes a convex surface on one of the transfer tube and the at least one hollow spoke and a cylindrical surface on the other of the transfer tube and the least one hollow spoke.
 11. The gas turbine engine of claim 9, wherein the first connection includes a slip joint with one of the transfer tube and the at least one hollow spoke being received in the other of the transfer tube and the at least one hollow spoke.
 12. The gas turbine engine of claim 9, wherein the first connection is configured to allow relative movement between the at least one hollow spoke and the transfer tube.
 13. The gas turbine engine of claim 12, wherein the second connection is configured to rigidly secure the transfer tube to the inner frame case.
 14. The gas turbine engine of claim 9, wherein the transfer tube includes a flange and a neck portion extending from the flange.
 15. The gas turbine engine of claim 14, wherein the transfer tube includes a passage with a circular cross section at a first end and one of a race track cross section, circular cross section, and elliptical cross section at a second end.
 16. The gas turbine engine of claim 9, wherein the inlet passage in the at least one hollow spoke is in fluid communication with a low-rotor cavity.
 17. A method of assembling a mid-turbine frame comprising: forming a first connection between at least one hollow spoke and a transfer tube; forming a second connection between the transfer tube and an inner frame case, wherein one of the first connection and the second connection is rigid and the other of the first connection and the second connection is non-rigid.
 18. The method of claim 17, wherein the first connection includes a spherical joint and the spherical joint includes a convex surface on one of the transfer tube and the at least one hollow spoke and a convex surface on the other of the transfer tube and the at least one hollow spoke.
 19. The method of claim 17, wherein the first connection includes a slip joint with one of the transfer tube and the at least one hollow spoke being received in the other of the transfer tube and the at least one hollow spoke.
 20. The method of claim 17, wherein the second connection is configured to rigidly secure the transfer tube to the inner frame case. 